Nuclear transfer involves the transplantation of living nuclei from typically embryonic cells to unfertilized eggs. The early research on vertebrates was performed in amphibians. The embryonic frog blastomere cells were separated and the nuclear material was introduced into frogs' eggs which had been enucleated. See Transplantation of Living Nuclei From Blastula Cells into Enucleated Frogs Eggs, R. Briggs and T. J. King, Proceedings of the National Academy of Sciences, Volume 38, pages 455-463, 1952. Further experimentation was performed on amphibians and amphibian eggs to determine if nuclear material from adult frog somatic or germinal cells could be transplanted to eggs and develop into a normal larva. Development and Chromosomal Constitution of Nuclear-Transplants Derived from Male Germ Cells, M. A. Berardino and N. Hoffner, Journal of Experimental Zoology, Volume 176, pages 61-72, 1971. There is a great degree of uncertainty as to when the genetic material of a cell can no longer be reprogrammed, which limits the stage of development of donor nuclear material.
Transplantation of nuclear material in mammals has proved very difficult to achieve which is in part due to the microsurgical techniques on mammalian embryos and eggs. The microsurgical techniques can be destructive to delicate cell structure which damages the cell material used in the later stage of the transplantation procedure. An alternative procedure is to deliver the nuclear material to a recipient egg by fusion of an intact cell or a karyoplast consisting of a nucleus surrounded by a piece of plasma membrane to the egg. The manipulation to isolate a karyoplast is performed in the presence of cytochalasin B. Methods and Success of Nuclear Transplantation in Mammals, A. McLaren Nature, Volume 109, June 21, 1984. Also, once the nuclear material has been transplanted to a recipient a mammalian egg there is fusion of the cellular material to produce a new viable embryo. The fusion can be aided or induced with virus or electro-field induced. However, the conditions for fusion are not predictable. Electric Field-Induced Cell-to-Cell Fusion, U. Zimmermann and J. Vienke, Journal of Membrane Biology, Volume 67, page 165-182 (1982).
Nuclear transplantation in higher mammals has been attempted. Successful nuclear transplantation and cell fusion was achieved for sheep embryos when individual blastomeres from 8 and 16 cell embryos were used as the nuclear donors into enucleated or nucleated halves of unfertilized eggs. Nuclear Transplantation in Sheep Embryos, S. M. Willadsen, Nature, Volume 320, pages 63-65, March 1986. Nuclear transplantation has been attempted in bovine embryos, however, the embryos developed only 43 days out of a nine month typical gestation period. Nuclear Transplantation in Bovine, J. M. Robl, R. Prather, W. Eyestone, F. Barnes, D. Northey, B. Gilligan, and N. L. First, Theriogenology, Volume 25, No. 1, January 1986. Successful nuclear transplantation and embryo development in higher mammals has great implications in breeding.